Chapter 5 - Looking Inside Materials Flashcards
Explain Lord Rayleigh’s oil drop experiment.
- Measure the diameter d of the oil drop
- Calculate the radius r
- Place the oil drop on still water to observe it spreading
- Measure the diameter of the patch of oil D after it has spread
- Calculate the radius R of the oil patch
- Use 4/3 pi r^3 = pi R^2 h
- Rearrange for h = 4r^3 / 3R^2
- h represents the thickness of the layer which allows us to measure the largest possible length of an oil molecule
To what order is the diameter of an atom of?
10^10 m
What can atomic force microscopes (ATMs) and scanning tunnelling microscopes (STMs) be used for?
Showing individual atoms.
What can scanning electron microscope (SEMs) be used for?
Showing larger scale structures.
What technique can be used to tell us more about the arrangement of atoms beneath the surface of a material?
X-ray diffraction crystallography.
What does a crystalline structure mean?
That the individual particles are arranged in a regular pattern over distances many times the spacing between the appearance of the material. e.g. metals
What is a dislocation?
A defect in the regular structure of a crystal or crystalline region of a material. Dislocations in metals are mobile and make metals ductile.
What is an amorphous structure?
A structure that has no order e.g. the microscopic structure of glass. The atoms in glass form strong bonds with one another to make up a rigid structure without any regularity.
What is a polycrystalline structure?
A structure composed of tiny crystal grains. Within each grain the material shows an ordered structure but the orientation of each individual grain is random.
What is a grain boundary?
The line along which grains meet in a crystalline structure.
What is an alloy?
An alloy is a material composed of two or more metals, or a mixture of metals and other materials.
Why are alloys often stronger than pure metals?
The mixture of different atoms sizes. The additional material’s atoms can pin down dislocations in the metal structure, making slippages between layers of atoms more difficult.
What structure state will liquids result in if it is cooled rapidly?
Amorphous
What structure state will liquids likely result in if it is cooled in a slow controlled manor?
Pure crystalline
Explain the process of crack propagation in glass.
- > Glass rod has a tiny scratch on surface.
- > The glass becomes elastically strained.
- > At the tip of the crack, two neighbouring atoms are pulled apart.
- > The next two atoms are pulled apart, and the next and the next…
- > The crack moves through the material like a zip being undone, propagating through the material.
How do metals resist cracking?
Because they are ductile. Under stress, the metal deforms plastically in the crack region making it broader, reducing the stress around it.
Give a similar and different property of glass and steel.
- > Have very similar breaking strengths.
- > Steel is much tougher.
What are the three types of bonds between atoms?
Covalent, ionic, and metallic
What is a further property that ionic compounds and ceramics have?
They are directional meaning the atom is locked in place and cannot slip, making them hard and brittle.
Explain a metal’s microscopic structure.
Metals such as gold have non-directional bonds. The atoms in metals are ionised and negatively charged free electrons more between the ions, acting as a glue. The bonds are strong so metals are stiff. The ions can slip so metals are ductile and tough.
Up to what strain do metals behave elastically?
About 0.1%
Up to what strain do polymers behave elastically?
About 1%
Give properties of the polymer, Polythene.
- > “Floppy” - free to rotate around its bonds
- > Strong bonds - difficult to break
- > Strong and flexible
What are cross-linkages?
Where polymers are ‘tied together’ at regular intervals along the chains, in order to produce a stiffer polymer - as the rotation / unfolding of the chains of molecules is difficult.
